remove old error and add an explanation

Author: nikomatsakis

src/test/ui/impl-trait/needs_least_region_or_bound.rs

@@ -1,10 +1,22 @@
+// run-pass
+
 use std::fmt::Debug;
 
 trait MultiRegionTrait<'a, 'b> {}
 impl<'a, 'b> MultiRegionTrait<'a, 'b> for (&'a u32, &'b u32) {}
 
 fn no_least_region<'a, 'b>(x: &'a u32, y: &'b u32) -> impl MultiRegionTrait<'a, 'b> {
-//~^ ERROR ambiguous lifetime bound
+    // Here we have a constraint that:
+    //
+    // (x, y) has type (&'0 u32, &'1 u32)
+    //
+    // where
+    //
+    // 'a: '0
+    //
+    // then we require that `('0 u32, &'1 u32): MultiRegionTrait<'a,
+    // 'b>`, which winds up imposing a requirement that `'0 = 'a` and
+    // `'1 = 'b`.
     (x, y)
 }